1. Field of the Invention
The present invention generally relates to optical scanning apparatuses and more particularly, to an optical scanning apparatus for converging a light beam carrying image information on a scanned medium, using an image-forming mirror. The optical scanning apparatus of the present invention is suitably used in a write optical system of laser printers, image forming apparatuses, measuring equipment and testing devices.
2. Description of the Related Art
Various optical apparatuses have been proposed for converging a light beam deflected by a deflector on a scanned medium, using an image forming mirror. Many of these optical scanning apparatuses are provided with facet-inclination-correction function for correcting shifts of points of beam convergence occurring as a result of an inclination of a facet of the deflector. For example, Japanese Laid-Open Patent Application No. 1-200221 provides for facet inclination correction by providing a facet-inclination-correction toroidal lens in an optical system. Japanese Laid-Open Patent Application No. 6-123844 provides for facet inclination correction by providing a facet-inclination-correction elongated cylindrical element in an optical system. These approaches are designed to eliminate the scan line shift due to the inclination of the polygon facet of the deflector by placing the polygon facet and the scanned surface in a conjugated arrangement.
In an alternative approach, Japanese Laid-Open Patent Application No. 4-194814 discloses an image-forming mirror provided with a facet-inclination-correction function.
However, providing a facet-inclination-correction elongated cylindrical element (No. 6-123844) or a facet-inclination-correction (elongated) toroidal lens (No. 1-200221) in an optical system increases the cost of the optical scanning apparatus and decreases the flexibility in designing the apparatus.
According to the approach disclosed in Japanese Laid-Open Patent Application No. 4-194814, it is difficult to provide a satisfactory separation of optical paths. If the separation of optical paths is small, the flexibility in designing the apparatus suffers greatly.
In order to ensure a large angle of separation, a half mirror must be used. However, use of a half mirror results in a relatively high cost and a relatively large space required for the apparatus. Another disadvantage in using a half mirror is that a beam intensity drops to approximately 1/4 after passing through the half mirror.
Providing a large separation of optical paths without using a half mirror, however, results in a relatively large scan line bow, necessitating a reduction in the angle of field, as is well known in the art. If the angle of field is reduced, the beam has to travel a relatively long distance in order to scan an area of a desired size. In order to ensure a satisfactorily large angle of field, the image-forming mirror must be machined so that it is deflected in a sub-scanning direction.
As described above, the optical scanning apparatuses according to the related art have various inherent problems in terms of their cost and production.
Japanese Laid-Open Patent Application No. 64-78214 provides alternative optical scanning wherein a light beam is reflected multiple times in a reflective optical system comprising a spherical concave mirror and a reflecting mirror arranged opposite to each other. However, the optical scanning system disclosed in Japanese Laid-Open Patent Application No. 64-78214 does not provide for facet inclination correction for correcting a scan line shift due to an inclination of a facet of a deflector such as a polygon mirror. Accordingly, a variation in scanning pitches occurring as a result of an inclination of a deflector cannot be prevented.